anti-compton BGO detector

Transcription

1 1 2 3 Q β - measurements with a total absorption detector composed of through-hole HPGe detector and anti-compton BGO detector 4 5 Hiroaki Hayashi a,1, Michihiro Shibata b, Osamu Suematsu a, Yasuaki Kojima c, 6 Kiyoshi Kawade a, Akihiro Taniguchi d 7 a Graduate School of Engineering, Nagoya University, Nagoya , Japan 8 b Radioisotope Research Center, Nagoya University, Nagoya , Japan 9 c Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima , 10 Japan 11 d Research Reactor Institute, Kyoto University, Kumatori , Japan 12 1 Corresponding author. hayashi.hiroaki@b.mbox.nagoya-u.ac.jp; FAX:

2 13 Abstract 14 A total absorption detector, which is composed of a through-hole type HPGe 15 detector coupled with a surrounding annular anti-compton BGO detector, has been 16 developed for β -decay energy (Q β ) measurements of nuclei far off the β-stability line. 17 This detector can measure radiations with an almost 4π solid angle by putting 18 radioactive sources in the middle of the HPGe detector and also can suppress the 19 scattered photons with the anti-compton BGO scintillation detector. The systematic 20 uncertainty was determined to be 30 kev by measuring 19 nuclei having Q β s between 3 21 and 8 MeV by means of conventional square root plot analysis. The Q β s of 22 mass-separated fission products 147 La and 148 La were successfully determined to be (40) and 7732(70) kev, respectively. 24 Keywords : Q β ; total absorption detector, HPGe detector, anti-compton BGO scintillator, U(n,f), 147 La, 148 La, on-line mass separator 26 PACS : Mc; s; Ec Introduction 29 Atomic masses of unstable nuclei are fundamental and important physical quantities 30 related to nuclear structure and nucleosynthesis in astrophysics. Moreover, the masses 2

3 31 of fission products are important for the evaluation of decay heat in nuclear power 32 plants. Nevertheless, most β - -decay energies (Q β ) of nuclei far off the β stability line 33 have not been determined experimentally due to the low intensities of available beams. 34 Available theoretical mass values and the systematics have uncertainties of at least kev. It would be meaningful to determine the values experimentally with smaller 36 uncertainty, even as high as 100 kev. Presently, ion-trap methods coupled with on-line 37 mass separators (ISOL) are used at various facilities. Alternately, Q β measurement is 38 also considered to be a precise method. However, in Q β measurement by applying β-γ 39 coincidence method, information on a decay scheme is necessary. In the nuclei of 40 interest, and especially in newly observed isotopes, most of their decay schemes are not 41 known, making β-γ coincidence measurement quite difficult. On the contrary, a total 42 absorption detector can be extremely effective because the Q β can be determined 43 independently of the decay scheme information. This is because the energies of the β 44 and γ rays can be summed up to the Q β, and the end-point of the measured spectrum 45 corresponds to a Q β. It means the Q β can be determined independently of the decay 46 scheme information. Hence, a total absorption detector composed of twin large volume 47 BGO scintillators has been developed and installed in two ISOL facilities (KUR-ISOL 48 and Tokai-ISOL). The Q β s of fission products of 235 U(n,f) or 238 U(p,f) including new 3

4 49 isotopes have been successfully determined independently of the decay scheme 50 information [1-4]. This total absorption detector has higher efficiency but less energy 51 resolution in comparison with the Ge detector, so the deduced Qβs include uncertainties 52 of 60~100 kev. 53 In order to determine Q β s with higher accuracy and reliability, another total 54 absorption detector composed of HPGe and BGO scintillation detectors has been 55 recently developed. The efficiency of this total absorption detector is less than that of 56 the BGO total absorption detector [1], but the energy resolution is much higher. 57 Moreover, energy calibration can be carried out up to the high energy region by the 58 observed γ-rays (e.g. prompt γ-rays). Consequently, the deduced Q β s are much more 59 reliable. 60 The performance of the detector, namely that it could measure Q β s with 61 systematic uncertainties of within 30 kev under good statistics, was confirmed by 62 measuring the well-evaluated Q β s between 3 and 8 MeV using conventional square root 63 plot analysis. Finally, the Q β of 147 La and 148 La that were separated from fission 64 products using an on-line separator were proposed to be 5366(40) and 7732(70) kev, 65 respectively. 66 4

5 67 2. Experimental 68 As shown in Fig. 1, the newly developed detector is composed of two detectors. 69 One is a custom-made HPGe detector in which the large volume single crystal 70 (84mm φ 90mm t ) has a 20mm φ diameter through hole in the center. This allows 71 radiation to be measured with an almost 4π solid angle and to be measured energy sum 72 of β-ray and γ-ray following the β-decay. The Ge crystal is covered with aluminum 73 housing and the thickness of the well is 0.4 mm in order to reduce the energy loss and 74 absorption as low as possible. The other detector is the 25mm thick anti-compton BGO 75 detector which surrounds the HPGe detector. The energy resolution of the HPGe 76 detector was approximately 2.5 kev at the 1332 kev γ-ray. Measurements were taken as 77 follows: a singles spectrum using the HPGe detector and a coincidence spectrum with 78 the BGO detector were measured simultaneously. Here, coincidence events correspond 79 to incomplete energy absorption events, namely Compton components of γ-rays or 80 bremsstrahlung photons associated with β-particles. By subtracting the coincidence 81 spectrum from the singles spectrum, total absorption events can be extracted. Therefore, 82 the deduced spectrum principally corresponds to the superimposed spectrum of a fully 83 absorbed β-ray, and the end point of the spectrum corresponds to the Q β. 84 In practice, β-ray spectra are distorted by the energy distribution resulting from 5

6 85 energy straggling by the Al housing, a dead layer including some materials on the 86 surface of the crystals. Hence, in order to determine the effective energy loss 87 experimentally, many nuclei having well-determined Qβs were measured, and the 88 measured end point energies were compared to their evaluated values. Nine 89 radioisotopes 27 Mg, 38 Cl, 42 K, 52 V, 56 Mn, 72 Ga, 90 Y, 139 Ba and 142 Pr were prepared by the 90 (n,γ) reactions at the pneumatic tube facility in the Kyoto University Reactor (KUR) and 91 ten fission products of Rb, 93,95 Sr and Cs were provided by the on-line mass 92 separator at the KUR (KUR-ISOL) [e.g. 5, 6]. On the (n,γ) experiment, radioactive 93 sources in liquid form were dropped on thin filter paper. On the ISOL experiment, the 94 mass-separated radioactive beams were implanted into a thin Mylar tape, and the 95 sources on the tape were moved periodically with predetermined time interval from the 96 collecting port in a vacuum chamber to the measuring position in the air using 97 differential pumping. The tape was computer-controlled and the reproducibility of the 98 source position was within 1 mm. On the ISOL experiment, the detector was shielded 99 from the background neutrons with 5 cm lead blocks, 5 mm thick 40% boron-doped 100 rubber sheets and 20 cm thick paraffin blocks. These shields reduced the background 101 from 830 to 120 cps, approximately. This background rate was nearly constant during 102 the measurements. In both experiments, the source preparations and measurements were 6

7 103 iterated many times in order to obtain sufficient counting statistics. The total counting 104 rate was kept below 1.5 kcps to reduce pulse pile-up. Energy calibration was carried out 105 with 24 Na and capture γ-rays of the surrounding materials such as Fe, H, and N Results 108 In this paper, the results obtained through conventional analyzing method, 109 namely square root plots method, is described. The total absorption spectrum was 110 deduced from the singles spectrum by subtracting the coincidence spectrum multiplied 111 by a factor of 1.4 for every nucleus. This factor corresponds to the inverse of the 70% 112 solid angle of the BGO scintillation detector and it was consistent with not only the 113 measurements, but also with the simulations by the Monte Carlo code GEANT4 [8] for 114 mono-energetic γ-rays. The background spectra in singles and coincidences were also 115 subtracted from the each measured spectrum. Two typical total absorption spectra of Sr(Q β = 4.2 MeV) [7] and 92 Rb(Q β = 8.2 MeV) [7] are shown in Fig. 2(a) and (b), 117 respectively, together with singles and coincidence spectra. Here, the spectrum was 118 analyzed with the energy bin of 20 kev in convenience. Generally, in energy spectra 119 measured with Ge detector, the energy can be determined within the uncertainty of 1/5 120 channel or much better, in this case 4 kev, under the good statistics condition. The 7

8 121 energy broadening represented as the FWHM of response for 3-8 MeV mono-energetic 122 electrons, which occurs mainly by the housing of Aluminum well, is evaluated to be kev by the Monte Carlo simulation GEANT4, approximately. Moreover, as 124 mentioned later, the uncertainty of the analysis is evaluated to be 30 kev practically. 125 Therefore, the uncertainty originated from the bin does not have influence on the 126 analysis, and the energy bin of this 20 kev is enough to analyze the spectra with much 127 better precision than that of the BGO detector. 128 The square root plot method is often the preferred method of Q β analysis for 129 measurements with a plastic scintillation detector and can also be applied to the spectra 130 measured with the total absorption BGO detector [1, 2]. As shown in the inset of Fig. 2, 131 the square root plot of the spectrum shows an almost straight line near the end-point. 132 It suggests the distortion of the response for the electrons does not have influence so 133 much near the end point energy at least 500 kev, and the root plot analysis is enough to 134 analyze the measured total absorption spectrum in this detector. The negative counts 135 in square root plots show putting the minus sign when the counts become negative after 136 subtracting the coincidence spectra. The fact that the counts scatter around zero in high 137 energy background region after subtracting means the spectra were subtracted properly. 138 The spectrum also shows tailing to the high energy side at the end-point owing to 8

9 139 energy broadening. In the root plot analysis, the region of interest for analysis was 140 chosen to be about 500 kev below 100~200 kev from the end-point of each spectrum in 141 order to exclude this tailing effects. This method was employed for all spectra 142 independently of each decay scheme. 143 The comparison between the deduced end-point energies (E β-max ) obtained 144 through energy calibration carried out with γ-ray peaks and the literature Q β values [7] is 145 shown in Fig. 3. The effective energy loss by this analyzing method was experimentally 146 determined to be 180 kev between 2 and 8 MeV with an uncertainty of 30 kev. The Q β 147 of newly measured nuclei can be deduced by adding this effective energy loss to the 148 E β-max of the spectra. The origin of the energy loss was evaluated as follows. According 149 to the Monte Carlo simulation for mono-energetic electrons, the energy loss by Mylar 150 tape was evaluated to be 20 kev, approximately. So, that by 0.4 mm t Al housing and 151 dead layer including some materials on the crystal well was deduced to be 160 kev, 152 approximately. On the other hand, that by only 0.4 mm t Al housing was evaluated to 153 be 130 kev, approximately, then, that by dead layer including surface materials was 154 evaluated to be 30 kev. This effective energy loss for the (n,γ) experiment was almost 155 the same as that for the ISOL experiment. The uncertainty obtained through the 156 conventional analysis is comparable with, or better than, that of the BGO total 9

10 157 absorption detector [3]. 158 The Q β of some nuclei around A=150 were measured with the detector at the 159 KUR-ISOL. The periods for collection-measurement of the tape transport system for La (T 1/2 =4.0 s [9]) and 148 La (T 1/2 =1.4 s [9]) were set at 8 s 8 s and 3.4 s 3.4 s, 161 respectively, to reduce each daughter activity. The total measurement period for each 162 nucleus was longer than 15 hours. As shown in Fig. 4(a) and (b), the E β-max of 147 La and La were analyzed to be 5186 kev and 7552 kev, respectively, by means of the square 164 root plot as described above. Finally, after adding the effective energy loss of 180 kev, 165 the Q β s were deduced to be 5366(40) and 7732(70) kev for 147 La and 148 La, respectively. 166 The uncertainties were derived from the effective energy loss and each statistic. The 167 deduced value for 148 La has a relatively larger uncertainty which consists in the 60 kev 168 statistical one of 60 kev and the systematic one of 30 kev, approximately Discussion 171 The results and the previously proposed values are summarized in Table 1. The 172 value determined for 147 La is not in agreement with the previously reported values of (55) kev [10] and 5150(40) kev [11] over their uncertainties. However, it is close 174 to the value of 5150(40) kev. The value of 4945(55) kev [10] was adopted by Audi et al. 10

11 175 in The preliminary proposed value of 5370(100) kev [12], which was measured 176 with the BGO total absorption detector, is in good agreement with the present result. 177 The last evaluation in 2003 [7]; 5180(40) kev; seems to take ref. [11] into account 178 intensively, but not ref. [12]. Anyhow, the value of 4945(55) kev is quite small 179 compared to the present result. 180 Similarly, the result for 148 La is also in agreement with the result that was 181 measured with the BGO total absorption detector [12]. Previously, three experimental 182 values of >5862(100) [13], 7255(55) [14] and 7650(100) kev [12] were proposed. As 183 described in ref. 14, the value of 5862 kev [13] were re-evaluated to be 7310(150) kev 184 from the information of decay study. In the previous systematics in 1995 [15], the value 185 of 7262(50) kev was proposed. It seems to stand on the ref. 14 only. The last 186 systematics of 7260(50) kev for 148 La by Audi et al. [7], seems to exclude the value of (100) kev [12]. In the case of the neutron-rich La isotopes, it had proposed that 188 there is disagreement in the two neutron separation energies (S 2n ) between the 189 experimental values [12] and the systematics determined by Audi et al. [15]. [16] 190 Recently, Clark et al. [17] also proposed that the experimentally determined atomic 191 masses of 147,148 La by Canadian ion trap were more than 400 kev larger than those in 192 Audi et al. [15]. These results are consistent with the present results. Such large 11

12 193 differences might indicate some nuclear effects. Spectroscopic study is necessary to 194 further understanding of the nuclear structure. As mention above, the systematics 195 strongly depends upon the experimental values. It means the experimental values are 196 important for the proposition of reliable systematics. In further analysis taking 197 accounts of the energy broadening owing to energy straggling, the tailing region in high 198 energy side could be included in the analyzing region, and then the uncertainties will be 199 expected to be reduced. The results will be described in detail in a forthcoming paper. 200 In conclusion, a novel total absorption detector for Q β determination was 201 developed. It is composed of a large volume HPGe detector having a through-hole and a 202 BGO anti-compton detector. The ability of the detector to determine Q β with high 203 accuracy and independently of decay scheme information was demonstrated. The Q β s of 204 the 147 La and 148 La were determined to be 5366(40) and 7732(70) kev, respectively Acknowledgements 207 This work was carried out under the Research Collaboration Program of the Research 208 Reactor Institute, Kyoto University. This work was partially supported by the Ministry 209 of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (B), 210 No

13 211 References 212 [1] M. Shibata et al., Nucl. Instr. and Meth. A459 (2001) [2] M. Shibata et al., J. Phys. Soc. Jpn. 71 (2002) [3] H. Hayashi et al., E. Phys. J. A 34, (2007) [4] H. Hayashi et al., submitted in J. Phys. Soc. Jpn. 216 [5] A. Taniguchi et al., Nucl. Instr. and Meth. A351 (1994) [6] Y. Kawase and K. Okano, Nucl. Instr. and Meth. B37/38 (1989) [7] e.g. S. Agostinelli et al., Nucl. Instr. and Meth. 219 A506 (2003) [8] G. Audi and A.H.Wapstra, Nucl. Phys. A729 (2003) [9] Chart of the Nuclides 2004, Japanese Nuclear Data Committee and Nuclear Data 222 Center, Japan Atomic Energy Research Institute. 223 [10] M. Graefenstedt et al., Proc. of 5 th Int. Conf. Nuclei far from stability, NUFAST-5, , p.30, Edited by Ian S. Towner et al. AIP Conf. Proc [11] T. Ikuta et al., J. Phys. Soc. Jpn. 64 (1995) [12] M. Shibata et al., Proc. of the Third Int. Conf. on Atomic masses and Exotic nuclei, 227 Hämeenlinna, Finland, 2-7, July, 2001, p.479, Edited by J. Äystö et al. Springer. 228 [13] D. S. Brenner et al., Phys. Rev. C 26 (1982)

14 229 [14] M. Graefenstedt et al., Z. Phys. A336 (1990) [15] G. Audi and A.H.Wapstra, Nucl. Phys. A595 (1995) [16] O. Suematsu et al., KURRI Prog. Rep. (2001) 37. Published by the Research 232 Reactor Institute, Kyoto University. ISSN [17] J. A. Clark et al., Nucl. Phys. A 746 (2004) 342c. 14

15 234 Table 1 The comparison of the experimental and systematics Q β for 147 La and 148 La. Nuclide Q β Experimental Systematics Present Previous 1995 a 2003 b 147 La 5366(40) 4945(55) c 5150(40) d 5370(100) g 4945(55) 5180(40) 148 La 7732(70) 7310(150) e 7255(55) f 7650(100) g 7262(50) 7260(50) 235 a) taken from ref.15 b) taken from ref.7 c) taken from ref.10 d) taken from ref e) taken from ref.13 f) taken from ref.14 g) taken from ref.12 15

16 237 List of Figures Fig. 1 Schematic view of the total absorption detector. The size is indicated in mm scale Fig. 2 Measured spectra of mass-separated 93 Sr(a) and 92 Rb(b). The solid and broken lines indicate the singles and coincidence spectra, respectively. Dots indicate the total absorption spectrum which was obtained by subtracting the coincidence spectrum from the singles one multiplied by a factor of 1.4. The inset shows the square root plot of the total absorption spectrum. The region between two dotted lines indicates the analyzed region. It shows almost straight line near the end-point. The negative counts in square root plots show putting the minus sign when the counts become negative after subtracting the coincidence spectra. The counts scatter after subtracting around zero in high energy region. It means the spectra were subtracted properly Fig. 3 Effective energy loss of the detector. The differences between the determined end-point energies and the evaluated Q β s show the effective energy loss of the HPGe detector Fig. 4 The square root plots and the deduced E β-max of 147 La(a) and 148 La(b). Each region between dotted lines is adopted for analysis. 16

17 movable tape RI beam from ISOL Differential Pumping Photomultiplier Tube Ge mm t Al 25 BGO Cold finger

18 Counts per energy Sqrt(N) Singles Coincidence Total absorption (a) 93 Sr Energy(keV)

19 Counts per energy Sqrt(N) Singles Coincidence Total absorption (b) 92 Rb Energy(keV)

20 Residuals(Exp.-Ref.) (kev) Pr 90 Y 42 K 27 Mg 139 Cs 72 Ga 95 Sr 38 Cl 140 Cs 93 Rb 93 Sr 91 Rb Rb 142 Cs 56 Mn 52 V 141 Cs Q β in ref. (kev) -180 kev

21 Counts per energy Sqrt(N) Singles Coincidence Total absorption (a) 147 La Energy(keV)

22 10 5 Singles Coincidence Tota absorption Counts per energy 10 3 Sqrt(N) (b) 148 La Energy(keV)